The Nervous System Flashcards
What is our nervous system made up of?
-CNS:brain and spinal cord
-PNS:motor neurones, sensory neurones and receptor
Label the motor neurone
See notes
Should include
-axon terminals/synaptic bulbs
-cell body/ soma
-nucleus
-myelin sheaths
-nodes of Ranvier
-axon
-dendrites
Label the sensory neurone
See notes
Should include
-receptor cell
-myelin sheath
-axon
-cell body
What do we mean by the resting potential of a neurone?
The potential difference, normally -70mv, which is present across the resting neurone membrane, due to the fact that there is a build-up of positive Na+ ions outside the membrane
What is depolarization?
The reversal of the membrane potential from -70mv to +40mv due to sodium, Na+ ions diffusing inwards
What do we mean by the action potential?
The action potential is the depolarization and reversal of polarity that occurs across the neurone membrane when an impulse is being conducted
What do we mean by repolarization of the neurone membrane?
This is the return to resting potential again, (+40mv to -70mv) brought about by positive ions (this time potassium) diffusing back out of the axon
As part of repolarization, an overshoot known as hyperpolarization occurs (more negative than normal). This period is known as the REFRACTORY PERIOD. What do we mean by REFRACTORY PERIOD?
It is the time period during which depolarization and a further action potential cannot occur
What is the importance of the REFRACTORY PERIOD in transmission of impulses?
-It ensures that impulses travel in the right direction ie onto the next neurone and not backwards
-It ensures that the AP’s are kept separate from one another ie discrete
-It limits the number of action potentials that can be fired
What is the All or Nothing Law?
If a threshold level of depolarization is reached, then an Action Potential is set up and each action potential will be of the same magnitude. If the threshold isn’t reached, then there will be no AP
How is the impulse propagated down the axon in an unmyelinated neurone?
Waves of depolarization travel down the axon as a series of local circuits, resulting in the resting region immediately ahead becoming depolarised, as the region immediately behind, becomes repolarised (likened to a Mexican wave)
How is an impulse propagated in a myelinated neurone?
Action potentials “jump” from one Node of Ranvier to the next, due to these regions being the areas which are most permeable to the diffusion of the ions, and the fatty myelin sheath being mainly impermeable. This is known as SALTATORY conduction.
What factors speed up electrical impulses?
-Myelination speeds up conduction due to saltatory conduction and the insulation prosperities of the myelin itself
-Wider acorns speed up conduction as there is less leakage of ions (making it easier to achieve resting and action potentials)
-An increase in temperature leads to faster rates of ion diffusion, therefore faster impulse conduction
What is a Synapse
The junction between the end of one axon and the dendrite or cell body of the receiving neurone
On the arrival of an AP in the synaptic bulb, which ions enter and cause synaptic vesicles to fuse with the pre-synaptic membrane?
Calcium ions, Ca2+
By what process are the neurotransmitters released from the synaptic vessels and by what method do they cross the synaptic cleft/gap?
Exocytosis and Diffusion
Name the neurotransmitter released by most motor neurones and say how it gets across the synaptic cleft
Acetylcholine- it moves via diffusion
What happens when the neurotransmitters attach onto their complementary receptor sites on the post-synaptic membrane?
The sodium ion channels open and positive Na+ ions diffuse into the post-synaptic neurone
What is an EPSP (Excitatory Post Synaptic Potential?)
The depolarization that occurs due to positive Na+ ions diffusing into the post-synaptic neurone. The depolarization must reach a threshold value for the action potential to be propagated onwards.
What needs to happen to halt the firing of the action potentials in the post-synaptic neurone?
The neurotransmitter (often Ach, Acetylcholine) is released from the receptor sites when it is broken down by the hydrolytic enzyme, Acetylcholinesterase, into ethanoic acid and choline
There are lots of mitochondria in the synaptic bulb. What are they needed for?
The mitochondria are needed to provide ATP for the active uptake of the Ach breakdown products and also to provide ATP needed to recycle Ach and package it back into the synaptic vesicles.
What is an IPSP (inhibitory post-synaptic potential)?
This is the hyperpolarization that occurs in the post synaptic neurone, due to negative chloride, Cl- ions, entering the neurone. It acts to prevent AP’s being fired
When EPSP’s and IPSP’s both occur on one post-synaptic neurone, from different neurones, what term do we use to describe what determines the outcome?
SUMMATION (the IPSP’s and the EPSP’s add together and whichever is “greater” determines whether an AP gets fired)
Some insecticides work by inhibiting the enzyme acetylcholinesterase. What does this lead to?
Acetylcholine is not released from the receptor sites on the post-synaptic membrane, therefore continual depolarization and firing of Action Potentials occurs. The insect’s muscles are continually excited which leads to convulsions and death
How are synapses useful?
-They allow one way transmission of electrical signals
-They provide integration of information from inputs that come from many neurones
-They allow fatigue to happen due to the neurotransmitter supply becoming exhausted- this prevents over-stimulation
How do synapses ensure unidirectional impulse travel?
-The synaptic vesicles and neurotransmitters are only present in the pre-synaptic bulb
-The receptors for the neurotransmitters are only present on the post-synaptic membrane
What name do we give to the special synapse that occurs between the synaptic bulk of a motor neurone and a muscle cell?
The neuromuscular junction
What names do we give the
-pre-synaptic membrane
-post-synaptic membrane at the muscular junction
-motor end plate
-sarcolemma
What is the potential outcome of an impulse arriving at the neuromuscular junction?
Muscle contraction
If an action potential is generated at the neuromuscular junction, how does it travel through the muscle fibres?
Through a system of T-Tubules
Which ions are released into the muscle fibres, ultimately leading to contraction?
Calcium ions
